My understanding is that the term "user agent" has a precise meaning, at
least within the HTML and CSS specifications. It refers to any software
that processes HTML and CSS respectively. This would include braille or
speech-based web browsers, but exclude conventional screen readers.
However, software which accepts data from a browser via DOM and processes
the markup and style sheets in order to generate an appropriate braille or
audio rendering, might well be considered a user agent.
The page author guidelines are aimed at both end users who create web
content and at developers of authoring tools, file conversion software,
etc., which generate documents to be delivered via the Web.